This invention relates to a method of forming a corrosion resistant film (resist) on the surface of a metal plate, and more particularly to a method of forming a corrosion resistant film on a sheet-like or web-like surface of substrate composed of copper or copper alloy by using sensitizing solution composed of water-soluble high molecular compound such as polyvinyl alcohol (hereinafter referred to as "PVA"), casein, glue or the like and ammonium dichromate (hereinafter referred to as "ADC"). The method according to this invention is applied to photoetching of copper or copper alloy.
When PVA-ADC sensitizing solution composed of water-soluble high molecular compound (hereinafter represented by PVA) and ADC is used for forming a corrosion resistant film for photoetching mild steel material or invar steel to be used as shadow mask or the like, such problem as fog (unnecessary curing) caused by dark reaction or continuing reaction (curing without exposure) does not occur in practical use. That is, even in such a case that after degreasing the surface of mild steel material or invar steel, PVA-ADC sensitizing solution is applied to the surface of substrate and is dried, then after leaving for 4 days as it is (4 days leaving after coating) it is photographically printed, and further after leaving 2 days as it is (2 days leaving after printing) it is developed, the fog does not occur and as a result next etching process does not suffer from any inconvenience.
On the other hand, when applying said PVA-ADC sensitizing solution to a copper or an alloy containing copper, at the step of leaving after coating or leaving after printing, a dark reaction or a continuing reaction is taken place, and a fog is caused thereby, making the photoetching process difficult. Concerning this phenomenon of fog, when PVA alone or ADC alone is applied to the copper or copper alloy, the progress of the dark reaction is very slow, but when a sensitizing solution composed of PVA and ADC mixed with each other is applied to a substrate of copper or copper containing alloy, the dark reaction is immediately taken place and a corrosion resistant film is formed.
Such being the case, when applying a photoetching process to a copper or copper alloy, it is usual to apply either (I) PVA-ADC sensitizing liquid on condition that the leaving after coating or leaving after printing is not effected or (II) an organic solvent photoresist or the like of which catalytic reaction is slow on the copper or copper alloy (For example, KMER, KTFR produced by Eastman Kodak Company or FPR produced by Fuji Chemicals Industrial Co., Ltd.). Even when employing these methods, since a problem still remains in that the metal surface is coated with a metal oxide film or sticked by a very thin resist film. In order to remove them it is necessary to expose the metal surface to be worked as a pretreatment or preliminary process of etching process (hereinafter referred to as "preetch"). Particularly when the image line width of a printed pattern is so fine as to be 50 .mu.m and less, said preetch treatment is indispensable.
In addition, when applying photoetching to a substrate of copper or copper alloy material, there are following problems to be overcome:
(I) In the developing process by using PVA-ADC sensitizing liquid and without leaving after coating or leaving after printing, it is necessary to have a consecutive treating apparatus, and therefore,
1. The overall length of the equipment becomes longer.
2. When trouble occurs in the apparatus, it must be stopped, and besides since the substrate under the developing process may suffer from a fog caused by the dark reaction, an economic loss caused by such defective product is considerable, and in addition it is troublesome to start again the apparatus.
3. Once the process has been started, the operation must be continued up to the final step, and an inconvenience exists in that it is impossible to tempolarlily preserve or pool an intermediate product by each step of the developing process. Such inconvenience is considerable particularly in case of a substrate composed of web-like material.
(II) Meanwhile, in case of a developing process using an organic solvent photoresist,
1. The cost of photoresist is high, and since the developing solution is an organic solvent which is expensive being quite different from water, the manufacturing cost on the whole is increased.
2. So far as an organic solvent is used, there is the possibility of inflammation, and therefore it is necessary to take sufficient measures in view of security as well as industrial hygiene.